Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongation with minimum necking when formed within a limited temperature and strain-rate range. This characteristic, peculiar to certain metal and metal alloys, has been known in the art as applied to production of complex shapes. It is further known that at superplastic-forming temperatures, the same materials can be diffusion-bonded by forcing clean contacting surfaces together.
Many prior art processes and structures use diffusion bonding and superplastic forming, such as shown in: Hamilton et al., U.S. Pat. No. 3,927,817; Ko, U.S. Pat. No. 4,292,375; Rainville, U.S. Pat. No. 4,530,197; Cooper et al., U.S. Pat. No. 5,069,383; and Bottomley et al., U.S. Pat. No. 5,330,093 which must include a maskant or "stop off" material to prevent unwanted bonding, and Blair, U.S. Pat. No. 4,318,965; Violette et al., U.S. Pat. No. 5,129,787; Gregg et al., U.S. Pat. No. 5,330,092; Matsen, U.S. Pat. No. 5,420,400; and Gregg et al., U.S. Pat. No. 5,451,472 which disclose superplastically formed diagonally reinforced structures and the processes to construct the same.
As shown in Hayase, et al., U.S. Pat. No. 4,217,397, four sheets of superplastically formable material, such as titanium alloy can be used to provide a metallic sandwich structure. Generally, two or three contiguous work sheets are joined together by a distinct continuous seam weld in a pre-selected pattern, which determines the geometry of the structure of the core to be produced. An expandable envelope is formed by sealing the perimeter of the joined sheets. The joined and unjoined work sheets are then placed in a stacked relationship and contained in a limiting fixture or die. The space between the upper and lower limiting fixture members determines the height and shape of the sandwich structure that ultimately results. At least one of the work sheets is then superplastically formed against the other work sheet, to which it becomes diffusion-bonded to form the desired sandwich structure.
A particularly advantageous structure that can be formed is a four-sheet. structure (two face sheets and two core sheets) that ultimately results in two generally parallel face-sheets with perpendicular webs extending there between. The face sheets, are formed first and held in a proper final position inside forming dies in a hot press by pressure applied between the core sheets and the face sheets, while pressure is also applied between the core sheets to prevent them from bonding together. The webs are then formed by increasing the pressure between the two core sheets. The core sheets are welded together along seams there between, with at least one area of each seam where the weld is interrupted so that gas pressure can propagate between the welded core sheets. When pressurized during a superplastic forming operation, the spaces between the seams of the two core sheets expand into balloon-like structures until they contact the face sheets and can expand outwardly no further. Application of continuing pressure causes the balloon-like structures to assume square shapes with the seams being positioned halfway between the face sheets on what become perpendicular webs. The core sheet portions adjacent the webs ultimately are formed 90.degree. into contact with each other, and diffusion-bonded together into a single web structure. Additional core sheets can be included to form diagonal reinforcing ribs, and portions can be reinforced by suitably placed doubler strips and other reinforcements.
In the past, two approaches have been used to apply face sheet pressure to properly expand the face sheets against the hot die and retain them against the die as the core is expanded. One was welding a number of pressure tubes at the edges of the sheets for applying the pressure to each face sheet. The other was creating internal face sheet pressure ways by cutting sheets and welding selected areas. Both approaches are time consuming and lack reliability. Quite often, incomplete formings have been caused by leakage from weld cracks or gas passage blockages.
Therefore, there has been a need to improve the basic face sheet and expanded core sheet SPF/DB process so that the face sheets can be formed and retained reliably and easily.